Raise error if module is required but not found

Otherwise the error will just be silently ignored.
Search for library in proper build directory

The library will end up in the opm/json library in a build tree; we
want to be able to point the CJSON_ROOT variable to the root of the
build tree and have it pick up the libraries from there.
Search for build tree relative to this project

The CMake script is run in the source tree. We assume a certain
directory layout and that the cjson is available in a sibling
directory of this project. Thus, we can pick up a build tree without
having the library installed.
Use math library as a prerequisite (on Linux)

It is needed for pow() and floor() functions.
Look in lib64/ for x86_64 libs on RHEL(-like) distro
Set plural form of header dir. var.

Even though we only look for one directory, the convention is to always
have the full set of prerequisites in the plural form.
Correct documentation typo
Define uppercase version of found-variable

It doesn't really matter here because "cjson" is only letters, but as
a general rule we define the uppercase variant ourselves so that CMake
doesn't use its own (bizarre) case rules, and then set the lower case
explicitly afterwards.
Include standard CMake module to get error handling
Merge pull request #34 from rolk/034_cjson

Fix find module for cjson library